1. Field of the Invention
The present invention relates to a liquid crystal display driver for use in a display unit of a desktop electronic calculator (hereinafter referred to as calculator) or the like.
2. Description of the Prior Art
For duty-driving a liquid crystal display (hereinafter abbreviated LCD), it is necessary to apply a bias voltage so as to obtain a proper on-off effective value. In this operation, at least three voltages have been required inclusive of two intermediate level voltages in addition to a supply voltage. For example, in a dry battery type calculator, a driving operation is performed with 1/3 duty .multidot.1/3 bias or 1/4 duty.multidot.1/3 bias having two values of intermediate level voltage. The above 1/3 duty.multidot.1/3 bias is achieved by signals of the conventional waveforms shown in FIG. 7. Supposing now E=1.5 V, the V.sub.ON /V.sub.OFF ratio .alpha. becomes .sqroot.3.apprxeq.1.73. In a solar battery type calculator (hereinafter referred to as SB calculator), it is customary to perform a driving operation with 1/3 duty.multidot.1/2 bias having three values of a solar battery voltage, a doubled voltage of the battery obtained through a booster and an intermediate level voltage. In the former dry battery type calculator where intermediate level voltages are obtained by division through a bleeder resistor, the current is slight. However, in the latter SB calculator where the set current is as small as 1/2 to 1/3 of the bleeder current in the dry battery type, it is impossible to adopt a means for producing an intermediate level voltage by a bleeder resistor. Therefore, its power source is formed by the use of a booster equipped with two capacitors outside of an LSI. In the above structure, the number of required component parts is increased due to the necessity of a booster, rendering the circuit configuration complicated.
Meanwhile, with regard to another system for duty-driving the LCD at two voltages of a single power source without using such booster which causes the above-mentioned disadvantages, there is a conventional pulse control system that executes driving by pulses of the waveforms shown in FIG. 8 or 9. In the 1/2 duty pulses of FIG. 8: (a) shows a waveform H1 where h1 represents a selection period and h2 a half selection period; and (b) shows another waveform H2 where h2 represents a selection period and h1 a half selection period. The waveform so shaped as to apply a voltage during each selection period has an effective on-value in common, while the waveform so shaped as not to apply any voltage has an effective off-value.
When E=1.5 V, V.sub.ON =.sqroot.3/4.multidot.E=1.3 V and V.sub.OFF =.sqroot.1/4.multidot.E=0.75 V. Therefore the V.sub.ON /V.sub.OFF ratio e becomes .sqroot.3.apprxeq.1.73. Meanwhile, in the 1/3 duty pulse shown in FIG. 9, V.sub.ON =1.22 V and V.sub.OFF =0.87 V, so that .alpha.=1.41. Although it is possible to produce a 1/4 duty waveform in a similar way, the ratio .alpha. comes to be so small as 1.29. Since the contrast of the LCD becomes higher with increase of the ratio .alpha., it is customary in the calculator to adopt a system that ensures a greater value of .alpha. exceeding 1.73.
The number of signals required for driving the LCD elements can be reduced as the denominator in the LCD-driving duty factor becomes greater, in such a manner that 1/3 is superior to 1/2, 1/4 to 1/3 and so forth. Therefore, duty drive with such a greater value is desirable on condition that the same display quality can be achieved.
However, in the conventional structure mentioned above, 1/2 duty is the limit due to the value of a for pulse-driving the liquid crystal display in the calculator, and 1/3 duty is not employable with respect to the display quality or contrast. Meanwhile for LCD drive in the SB calculator, a 1/3 duty.multidot.1/2 bias system is adopted in most cases. In driving an 8-digit LCD, for example, the required signals are 27 in total. As compared therewith, at least 36 signals are required when using 1/2 duty pulses which consequently bring about an increase in the chip size of an LSI and also a larger number of package pins, thereby causing higher costs of production.